Reciprocal cooperative effects of multiple ligand binding to pyruvate kinase

The formation of multiple ligand complexes with muscle pyruvate kinase was measured in terms of dissociation constants and the standard free energies of formation were calculated. The binding of Mn2+ to the enzyme (KA = 55 +/- 5 X 10(-6) M; deltaF degrees = -5.75 +/- 0.05 kcal/mol) and to the enzyme saturated with phosphoenolpyruvate (conditional free energy) KA' = 0.8 +/- 0.4 X 10(-6) M; deltaF degrees = -8.22 +/- 0.34 kcal/mol) has been measured under identical conditions giving a free energy of coupling, delta(deltaF degrees) = -2.47 +/- 0.34 kcal/mol. Such a large negative free energy of coupling is diagnostic of a strong positively cooperative effect in ligand binding. The binding of the substrate phosphoenolpyruvate to free enzyme and the enzyme-Mn2+ complex was, by necessity, measured by different methods. The free energy of phosphoenolpyruvate binding to free enzyme (KS = 1.58 +/- 0.10 X 10(-4)M; deltaF degrees = -5.13 +/- 0.04 kcal/mol) and to the enzyme-Mn2+ complex (K3 = 0.75 +/- 0.10 X 10(-6)M; deltaF degrees = -8.26 +/- 0.07 kcal/mol) also gives a large negative free energy of coupling, delta(deltaF degrees) = -3.16 +/- 0.08 kcal/mol. Such a large negative value confirms reciprocal binding effects between the divalent cation and the substrate phosphoenolpyruvate. The binding of Mn2+ to the enzyme-ADP complex was also investigated and a free energy of coupling, delta(deltaF degrees) = -0.08 +/- 0.08 kcal/mol, was measured, indicative of little or no cooperativity in binding. The free energy of coupling with Mn2+ and pyruvate was measured as -1.52 +/- 0.14 kcal/mol, showing a significant amount of cooperativity in ligand binding but a substantially smaller effect than that observed for phosphoenolpyruvate binding. The magnitude of the coupling free energy may be related to the role of the divalent cation in the formation of the enzyme-substrate complexes. In the absence of the activating monovalent cation, the coupling free energies for phosphoenolpyruvate and pyruvate binding decrease by 40-60% and 25%, respectively, substantiating a role for the monovalent cation in the formation of enzyme-substrate complexes with phosphoenolpyruvate and with pyruvate.

Interaction of manganese with bovine prothrombin and its thrombin-mediated cleavage products

The binding of the paramagnetic metal, Mn(II), to bovine prothrombin and the thrombin-mediated cleavage products of prothrombin, i.e. fragment 1 and the prethrombin 1 has been investigated. Analysis of the Scatchard plots of the binding data reveals that prothrombin has two high affinity Mn(II) binding sites with a Kd of 1.2 +/- 1.0 X 10(-5) M and approximately two to three lower affinity Mn(II) sites with a Kd of 1.3 +/- 1.0 X 10(-4) M. Positive cooperativity in Mn(II) binding to prothrombin was observed for the strong sites. Fragment 1, the phospholipid-binding region of prothrombin, possesses two high affinity Mn(II) sites with a Kd of 2.2 +/- 1.0 X 10(-5) M and at least two lower affinity sites with a Kd of approximately 2.5 +/- 1.0 X 10(-4) M. Positive cooperativity was not observed for the binding of Mn(II) to fragment 1. Prethrombin 1 binds one Mn(II) with a Kd of 3.2 +/- 1.0 X 10(-4) M. Using the values of free Mn(II) concentration, as determined by EPR measurements and the observed enhancements of the water proton relaxation rates at various concentrations of Mn(II) and protein, the binary enhancement values (epsilon b) of the metal-protein complexes were obtained. The extrapolated values are 11 +/- 0.4 for the initial prothrombin-binding sites, and 10 +/- 0.3 for the tight binding sites of fragment 1. The unique epsilon b value obtained for prethrombin 1 was 5.3 +/- 0.7. When Mn(II) was used in a Factor Xa-metal ion-phospholipid system for activation of prothrombin, the rate of generation of thrombin was less than or equal to 5% of that obtained when Ca(II) was employed in this activation system. Addition of Mn(II) to the same activation system containing Ca(II) resulted in a marked decrease in the rate of thrombin generation, suggesting that Mn(II) probably competes for the same sites on prothrombin as Ca(II). In agreement with this is the observation that the Mn(II) sites on prothrombin could be displaced by Ca(II) at high concentrations of Ca(II).

Conformational changes required for pyruvate kinase activity as modulated by monovalent cations

The interaction of a series of alkylamines with muscle pyruvate kinase was investigated by kinetic and physical studies in order to understand the mechanisms by which certain monovalent cations can activate the enzyme and to define several of the important conformational changes necessary for catalytic activity. Monomethylammonium ion interacts with pyruvate kinase to activate the enzyme. Dimethyland trimethylammonium ions do not activate, but are competitive inhibitors against activating cations. Tetramethylammonium ion neither activates nor inhibits pyruvate kinase activity. When the enzyme is in the presence of monomethylammonium ion or dimethylammonium ion, a conformational change is observed by ultraviolet difference spectroscopy. This conformational change is similar to that observed with other activating cations and appears to be a necessary but no sufficient conformational change in the formation of an active complex. The interaction of the substrate phosphoenolpyruvate with the pyruvate kinase-Mn2+ complex in the presence of these cations was studied by water proton relaxation rate measurements. The affinity of the enzyme-Mn2+ complex for phosphoenolpyruvate is decreased by a factor of 5 in the presence of any of the alkylamines compared to the affinity measured in the presence of K+ or NH4+. No change in the Km of phosphoenolpyruvate is observed however when it is measured in the presence of monomethylammonium ion, suggesting that the decrease in affinity for the substrate is not the reason for lack of enzymic activity. The conformation of the ternary enzyme-Mn2+-phosphoenolpyruvate complex about the bound Mn2+, as reflected by the enhancement values (epsilont) measured, differs depending upon the nature of the monovalent cation. The epsilon t values measured in the presence of the alkylamines are larger (epsilont - 5.7 +/- 0.2) than those measured in the presence of K+ or NH4+ (epsilont = 1.9 +/- 0.1).